David Cameron hit back at "totally inappropriate" criticism of his plans for tougher curbs on migrants by an "unelected" Brussels official as he sought support for his stance at a summit of fellow leaders.
The Prime Minister revealed that he collared European Commission president Jose Manuel Barroso at the Lithuania gathering to complain about the intervention by employment commissioner Laszlo Andor.
Mr Andor launched an outspoken attack on the Government''s benefit crackdown, warning Britain would be seen as "nasty" if it pushed ahead with the measures which he called "an unfortunate over-reaction".
Five weeks before Romanian and Bulgarian nationals get new rights to live and work in the UK, Mr Cameron has set out radical plans to overhaul welfare rules, including stopping new arrivals from the EU receiving out-of-work benefits for their first three months in the country.
He says he wants to send a clear signal to would-be migrants that Britain is not a "soft touch".
But he is also pushing for wider EU reforms such as limiting the free movement of workers - seen as a fundamental principle of the EU - from new member states until they hit a certain level of GDP per head.
Over dinner in Vilnius he told EU and east European leaders it was impossible to press ahead with welcoming new countries to the Union "without addressing abuse of free movement and how it works in future", Number 10 said.
Germany and France are among other key EU member states taking action on the benefits issue.
Mr Cameron wrote on Twitter that he had raised Commissioner Andor's comments with Mr Barroso and told him it was "totally inappropriate for unelected officials to complain about legitimate concerns".
A Number 10 spokesman said the PM made clear that such "behaviour is inappropriate for an official paid for by British and other European taxpayers.
"When elected European leaders are raising real and substantive concerns about an issue affecting people it is not for officials to dismiss those without any discussion," he said.
Under the new measures, EU nationals will only be able to claim out-of-work benefits for a maximum of six months unless they can prove they have a genuine prospect of employment.
New migrants will not be able to claim housing benefit immediately.
Those found begging or sleeping rough could be deported and barred from re-entry for 12 months unless they can show they have a proper reason to be in the UK, such as a job.
Foreign Secretary William Hague joined the criticism of the Commissioner's "nasty" jibe and insisted the UK's concerns were shared by many countries.
"I don't think we should accept that word at all. The United Kingdom is one of the most generous, open-hearted nations on earth," he told Channel 4 News.
"But we have to make it clear, and other European countries want to make it clear, that where there is an entitlement to free movement for work across the European Union, that is for work, it is not for claiming the benefits of another country.
"We are well within our rights to make that clear. There is nothing nasty about that.
"We have to do steadily more as we look at those ideas the Prime Minister raised as we discuss them with other countries.
"This is not just Britain being awkward in Europe; this is Britain giving voice to many people in Europe who say that freedom of movement for work is for work, not to claim more generous benefits in another country."
Meanwhile, the main business of the Eastern Partnership Summit threatens to prove problematic as plans for a major trade deal with between Ukraine and the EU appear to have been shelved under pressure from Moscow, although Georgia and Moldova are set to sign up to the agreement.